1. Field of the Invention
This invention relates to wireless communications. More specifically, this invention relates to pilot-assisted digital wireless communications.
2. Description of Related Art and General Background Call and Standby Modes
Wireless mobile communications involves communications between mobile units and/or between a mobile unit and a base unit. Mobile units typically operate in one of two principal modes. In call mode, the mobile unit is actively engaged in a communication with another unit. In standby mode, the mobile unit is not in use but is ready to receive incoming calls. Although a mobile unit in standby mode is usually not transmitting, it must still remain sufficiently active to detect and respond to signals that are directed to it, such as notifications of incoming calls.
It is desirable to reduce power consumption by the mobile units, thereby enabling them to operate for longer periods between battery replacement or recharging. One method of reducing power consumption is to interrupt the supply of power to circuits in the mobile unit that are not currently in use. Because a mobile unit in standby mode is not being used, for example, it would normally be advantageous to power down the display circuitry until a display is needed again.
Slotted Paging
In a typical wireless telephone application, the mobile unit receives information about incoming calls by monitoring a paging channel, which is a one-way link for communications from the base unit to the mobile units. When the base unit receives notice of a call destined for a particular mobile unit, it pages the mobile unit by broadcasting a paging signal over the paging channel. Included in the paging signal is an identifier associated with the mobile unit. When the mobile unit receives the paging signal and recognizes the identifier, it responds to the base unit in an appropriate manner on another channel (commonly called an access channel or access request channel) and the connection is initiated.
It has been recognized that a considerable portion of the power consumed by a mobile unit in standby mode is due to the RF circuitry, which receives the radio signal and outputs a data signal at baseband. Power consumption may be greatly reduced, and the time between battery recharges for a mobile unit in standby mode may therefore be significantly extended, by implementing discontinuous reception on the paging channel (a technique which is also called `slotted paging`). In one version of slotted paging, time is divided into consecutive slots of equal duration which are numbered in chronological order from 1 to N (where N is a counting number, and the slot numbering is restarted at 1 after N is reached). At least one slot number is assigned to each mobile unit, and the base unit is constrained to broadcast a paging signal to any particular mobile unit only during a slot whose number has been assigned to that mobile unit. As the mobile unit's RF circuitry may be powered down (i.e. the RF circuitry may be in an unpowered state) during most of the other slots, a significant power savings is thereby achieved.
In a slotted paging system, it is usually desirable to power up the RF circuitry at some moment prior to the start of an assigned paging slot, thus giving the circuitry a chance to stabilize by the time it is required to receive and output signals. This power-up moment is indicated in the timeline of FIG. 1 as point A.
In order for the receiver to receive valid symbols, it may be necessary to perform additional procedures once the RF circuitry has become stable. In a CDMA system, for example, acquisition of at least one (and preferably several) of the multipath instances of the received signal must be performed before the symbols in the signal may be identified. In FIG. 1, the start of the acquisition period is indicated by point B.
Digital wireless signals are usually encoded for various reasons (e.g., error correction, redundancy introduction and dispersal, encryption, etc.), and the received signals must therefore be decoded before interpretation may begin. Some of the circuits or algorithms used to decode these signals (such as Viterbi and other maximum likelihood decoders) should be initialized, typically with a preamble of received symbols, before they can begin to process data reliably. Once the acquisition process has completed and the receiver is ready to produce valid symbols, therefore, the decoder initialization period may begin (point C in FIG. 1).
At some subsequent moment, the mobile unit begins to receive the paging signal. This moment may occur as early as the start of the assigned paging slot (point D in FIG. 1), or it may be delayed depending, for example, on whether the base unit is occupied with other system activity. Eventually, the mobile unit will receive as much of the paging signal as it requires for proper interpretation, a moment indicated by point E in FIG. 1. It is possible for this moment to arrive before the entire paging signal is received, as the mobile unit only needs to interpret enough of the paging signal to know that no message is pending and may ignore the remainder. Possibly after a decoding delay, then, the interpretation of this portion of the paging signal will be completed (point F in FIG. 1).
In anticipation that no incoming message will be reported, it is possible to power down the RF circuitry (which event is indicated as point G in FIG. 1) as early as point E. Usually, however, the RF circuitry will be kept in a powered state until after point F (as shown in FIG. 1), in case interpretation of the paging signal should indicate that a response is required. In order to facilitate system synchronization, it may also be desirable to power the RF circuitry up or down only at moments having a substantially predetermined relation to a system reference time such as the slot boundary.
Existing cellular telephone systems that use the technique of slotted paging include those operating under the GSM and IS-95 standards. The parameters that define slotted paging under the IS-95 standard, for example, are presented in section 6.6.2.1.1 of TR45 Mobile Station-Base Station Compatibility Standard for Dual-Mode Wideband Spread Spectrum Cellular Systems (TIA/EIA/SP-3693 [to be published as TIA/EIA-95], TIA [Telecommunications Industry Association], Arlington, Va., 1997). Discontinuous reception in the GSM system is described in GSM Technical Specification 03.13 (version 5.0.0, March 1996, European Telecommunications Standards Institute [ETSI], 06921 Sophia Antipolis Cedex France).
Under the IS-95 standard, the slots in the paging channel are numbered from 0 to 2047 and each paging slot has a duration of 80 ms. A mobile unit has a slot number and a slot cycle index, and the number of slots n between the start of adjacent assigned slots may be expressed as a function of the slot cycle index i as EQU n=16.times.2.sup.i,
where i is a nonnegative integer. For a mobile unit having a slot cycle index of 0, for example, the starting times of each adjacent pair of assigned slots are separated by 16 slots (i.e. 1.28 seconds). Therefore, a mobile unit having a slot cycle index of 0 and the slot number 3 would be assigned to slots 3,19, 35, and so on up to slots 2019 and 2035.
For slot cycle indices of 1, 2, and 3, the respective periods of separation between the starting times of each adjacent pair of assigned slots are 32, 64, and 128 slots (or 2.56, 5.12, and 10.24 seconds). In North America, cellular telephones typically operate at slot cycle indices of 1, while a slot cycle index of 2 is more common for cellular telephones in Japan. The slot cycle index is a variable quantity, however, and it is also common for a cellular telephone to operate at slot cycle index 0 upon power-up until a good estimate of the relation between local and system oscillator frequencies is obtained. For another type of mobile unit, such as a remote data terminal, operation at a larger slot cycle index may be more appropriate; the IS-95 standard does not include an upper limit on the value of the slot cycle index.
Handoff Negotiation
At any one time, a mobile unit is usually connected to the system mainly through at least one particular base unit (i.e. the `home` base unit). Eventually, the mobile unit will travel beyond the range of the home base unit or will otherwise become unable to receive signals from this base unit any longer (e.g. because of signal path obstructions), and it win be necessary to establish a system connection (or to upgrade an existing system connection) between the mobile unit and another base unit (i.e. a new home base unit). It is desirable for a mobile unit to maintain a continuous connection with the system, thereby avoiding the necessity of rexecuting costly connection and authentication routines every time the mobile unit becomes connected to a different base unit. In order to allow the existing connection to continue, the system will typically negotiate a `handoff` of the connection from the old home base unit to the new home base unit.
Typically, the mobile unit will actively participate in handoff negotiations by monitoring the power of signals received from several base units in its vicinity, thereby obtaining information needed to choose the most appropriate new home base unit. Note that such active participation must continue even during standby mode if the mobile unit is to continue to receive paging signals. Therefore, a mobile unit in standby mode must not only receive and process paging signals, but it must also monitor the strengths of signals of nearby base units. It is desirable for the mobile unit to coordinate and overlap these two functions in order to minimize the period during which the RF circuitry is powered (hereinafter referred to as the `RF power period`).
Pilot Monitoring
In order to provide a power reference for the mobile units monitoring its signal, each base unit in a wireless telephone system will typically broadcast a substantially continuous beacon signal on some frequency, time, and/or code channel. In a system operating under the IS-95 standard, for example, the beacon signal is a pilot signal that may also serve as a phase reference for coherent demodulation of the paging signal. Typically, all of the various pilot signals will arrive on the same frequency channel as the paging channel, so that pilot and paging information may be extracted from the same received signal by applying the respective spreading and/or covering codes. Even in systems operating under the IS-95 standard, however, it is possible that one or more of the pilot signals will be transmitted on a different frequency channel, thus requiring the mobile unit to perform its monitoring tasks on more than one frequency channel.
In an IS-95 system, all of the pilot signals are spread by the same pseudonoise (PN) code. This code has a period of 32,768 chips (i.e. code symbols), and each pilot is distinguished from any other that may appear within its range by a unique offset that indicates the code starting point for the particular pilot. Five hundred twelve different offsets are defined, each being spaced at an interval of 64 chips from the two closest other offsets. The home base station supplies a list of offsets (i.e. the neighbor list) to the mobile unit for monitoring, wherein these offsets define the pilot signals of the neighbor base stations. Under the IS-95 standard referenced above (also called IS-95A), the neighbor list has 20 entries, while the list has 40 entries under the IS-95B revision (as defined, for example, in TIA/EIA-95B, "Mobile Station-Base Station Compatibility Standard for Dual-Mode Spread Spectrum Systems," SP-3693-1 published by TIA [Telecommunications Industry Association] March, 1999, Arlington, Va.). Although no maximum time between evaluations of each pilot is specified in the standard, as a matter of practice the mobile must reevaluate the strength of each pilot on the list within a short period of time or risk losing its connection to the system.
Clearly, the mobile unit has a significant amount of work to do in `standby mode.` Much of this work depends on information gained via the RF circuitry, and the mobile's task is therefore complicated by the desirability of reducing power consumption by minimizing the RF power period.
Searching and Search Result Processing
In order to monitor the active pilot (i.e. the pilot transmitted by the home base station) and the neighbor pilots (i.e. the pilots transmitted by the neighbor base stations), a mobile unit in an IS-95 system will have at least one searcher for detecting these pilots and measuring such parameters as signal strength and time of arrival. Searching for a particular pilot is typically done by correlating the received signal with the pilot PN code as shifted by (1) the particular pilot's known offset and (2) a series of smaller offsets to account for multipath delay effects. Parameters that may be used to define the search include the PN offset number, the number of lags in the correlation window, and a reference point from which the location of the zero lag position may be identified. Other parameters include the coherent and non-coherent integration lengths, which determine respectively the length of the window over which each correlation is performed and the number of consecutive windows whose correlation results are combined to obtain a single result. The search results may be processed, for example, to determine the received pilot's relative strength as compared to other received pilots and to adjust the parameters for the next search of this pilot.
Available circuit area in a mobile unit is typically extremely limited, and a mobile unit will usually have only one processing unit. FIG. 2 shows a functional diagram of a system for receiving paging information and performing pilot monitoring in which the paging and pilot signals are recovered from the same digital data signal as outputted by A/D converter 120. Power to RF stage 110 is controlled by RF power control 160 (which may be, e.g., a regulator with an enable terminal) in response to a command from processing unit 150 or possibly from another timing unit.
In order to avoid the need to extend the period during which RF stage 110 is powered (i.e. the RF power period), processing unit 150 performs two separate tasks in this period. One task is to supply parameters to the searcher 130 for conducting active searches (i.e. searches for the active pilot) and neighbor searches (i.e. searches for the neighbor pilots) and receive and process search results from searcher 130. The other task which processing unit 150 performs in this period is to control paging signal decoder 140 and interpret the paging message it outputs. Data memory 170 may be used for intermediate storage of search parameters and/or search results.
FIG. 3 is a flowchart showing a sequence of subtasks performed within the monitoring task by such a system, and FIG. 4 shows a timeline corresponding to FIG. 3 which displays a representative sequence of execution of the subtasks 210-240, where the subtasks are labeled as in FIG. 3. The period shown in FIG. 4 begins at a moment subsequent to the start of the acquisition period (point X being subsequent to point B in FIG. 1) and ends at the moment when the necessary paging information is interpreted (point F as in FIG. 1).
Note that processing unit 150 performs the monitoring task during this period only to reduce power consumption, and that it is possible to perform monitoring at any other time at the cost of powering the RF circuitry. On the other hand, it is essential for processing unit 150 to perform the paging task at this time, because the paging signal is not available at any other time. If the processing unit becomes too overloaded during this period to perform the paging task because of the additional processing required for the monitoring task, the mobile unit will be unable to detect an incoming page. Therefore it may be necessary to restrain the monitoring task by imposing a limit on the number of searches and/or the rate at which searches may be performed during this period. Such restriction is called `search throttling.`
Because the length of the RE power period must be limited to conserve battery power, the amount of time available to program and process searches is also limited. As the active pilot must be searched frequently in order to support the current link, the number of neighbors that may be searched in the time remaining in the RF power period corresponding to any particular assigned paging slot is therefore typically limited to less than half a dozen. A major consequence of this limitation is an incomplete state of knowledge within the system of the mobile unit's environment, which leads to poorer idle handoff performance and a requirement for longer and more frequent rehabilitative procedures to refresh the knowledge base. For example, when the quality of the knowledge of the environment drops below a predetermined level, the mobile unit will typically be required to enter a `link maintenance` condition, wherein it must keep the RF circuitry powered long enough to perform a new search for every neighbor on the list. Because link maintenance procedures increase power consumption significantly by extending the RF power period, it is desirable to reduce the number of times such procedures are required.